1. Field of the Invention
The present invention relates to a liquid crystal display device including a backlight capable of local dimming control.
2. Description of the Related Art
A mobile terminal such as a smartphone, a TV, or various monitors are often provided with a liquid crystal display device as an image display device. The liquid crystal display device includes a liquid crystal panel having liquid crystal elements two-dimensionally arrayed thereon the light transmittance of each of which is adjustable and a backlight (device) emitting a light to the liquid crystal panel.
In recent years, a liquid crystal display device has been known which includes a backlight having a plurality of illumination parts each of which can respectively illuminate each of a plurality of areas, the areas being obtained by dividing a display region of the liquid crystal panel, and performs local dimming control (also referred to as area control) to separately control luminance of each illumination part (see PLT1 to PLT 5). Particularly, in a case where a displayed image has a region locally low in tone and a region locally high in tone, contrast feeling can be improved by the local dimming control.
Such a liquid crystal display device that performs the local dimming control may improve the contrast feeling, while generating a new problem of image quality degradation. For example, a case is explained as an example, as shown in (a) portion to (c) portion of FIG. 21, where an image is displayed in two areas (area 0, area 1) adjacent to each other on the liquid crystal display device in which the image has a high tone portion in the area 0 and no high tone portion in the area 1.
An original image signal illustrated in (a) portion of FIG. 21 schematically shows a tone level (transmittance level of the liquid crystal panel) of the image to be displayed in the area 0 and area 1. A backlight luminance illustrated in (b) portion of FIG. 21 schematically shows luminance of the backlight controlled for each area. Luminance of an output image is that obtained by combining the transmittance of the liquid crystal panel controlled in response to the original image signal and the backlight luminance as illustrated in (c) portion of FIG. 21. At this time, the high luminance of the backlight in the area 0 causes a background portion thereof (portion surrounded by a dotted circle in the figure) to has higher luminance than the area 1, otherwise must have the same luminance as the area 1. In other words, controlling the luminance of the backlight for each area generates differences in brightness in a part which otherwise should have the same brightness to degrade the image quality.
Therefore, a liquid crystal display device described in PLT 6 corrects an original image signal illustrated in (a) portion of FIG. 22 as is illustrated in (b) portion of FIG. 22. Specifically, in order to suppress image degradation due to controlling the luminance of the backlight as illustrated in (c) portion of FIG. 22, an image signal for the area 1 is corrected to be made higher in the level than the original image signal. This, as illustrated in (d) portion of FIG. 22, the output image becomes an image having no image quality degradation correspondingly to the original image signal. In this way, appropriate computation of a signal for driving the liquid crystal panel and a signal for driving the backlight for each frame of the image to be displayed can achieve improvement of the contrast feeling and prevention of the image quality degradation at the same time.